A serial killer is a person who murders three or more people, in two or
more separate events over a period of time, for primarily psychological
reasons. There are gaps of time between the killings, which may range
from a few hours to many years. This list shows serial killers from the
20th century to present day by number of victims. In many cases, the
exact number of victims assigned to a serial killer is not known, and
even if that person is convicted of a few, there can be the possibility
that he/she killed many more. The complex nature of serial killers,
their crimes, discrepancies caused by geographic location and/or time,
and the investigations related to these persons results in difficulties
in organization and ranking. To address this, multiple categories have
been provided in order to more accurately describe the nature of certain
serial murders. This is not a reflection of an individual's overall rank
which may or may not vary depending on personal opinion concerning the
nature and circumstances of their crimes. The fourth column in the table
states the number of victims definitely assigned to that particular
serial killer, and thus the table is in order of that figure. The fifth
column states the number of possible victims the killer could have
murdered. Some of these crimes are unsolved, but were included because
they are the work of a serial killer, despite nobody being caught. This
list does not necessarily include war criminals or members of democidal
governments, such as Adolf Hitler, Hideki Tojo, Mao Zedong, Vladimir
Lenin, Joseph Stalin, or Pol Pot.